During operation, the boiler wall of a steam generator is subjected to great strain. In particular considering the tubes through which flows a medium material, it is necessary that materials be used that can be stressed accordingly. The boiler wall comprises a combustion chamber wall enclosing the combustion chamber of the boiler and an adjoining containing wall of a flue gas pass. The combustion chamber wall, as well as the containing wall, must be able to remove sufficient heat. In order to meet these requirements, the material that could be used for the walls is a martensitic steel alloy. However, these materials require the subsequent heat treatment, which is why it is desirable that a material be used that avoids the subsequent heat treatment. Therefore, ferritic, bainitic or bainitic-martensitic steel alloys are preferably used. Due to their chemical composition, a few of these alloys—even when welding takes place with pre-heating—tend to increased hardening when cooling from the welding temperature. Hereinafter, such alloys will be referred to as being “air-hardening”. In particular, the material T23 (7CrWVMoNb 9-6) or T24 (7CrMoVTiB 10-10) are used, these being defined and/or standardized by European Standard EN 10216, as well as by ASTM A213/A213M-09a, American Society for Testing and Materials (ASTM International; West Conshohocken, Pa. USA). Material T24 is referred to as standard 7CrMoVTiB 10-10. Based on the technical delivery conditions for seamless steel tubes for operating under pressure (DIN EN 10216-2) the carbon content of the components to be joined can be 0.05 wt %-0.10 wt % (wt %=percent by weight).
It has been found that tubes of such materials, in particular materials T24 and T23, are subject to damage during operation of the steam generator. The material hardens fully during the welding operation so that cracks form on the inside of the tube due to geometric irregularities in the root, due to the high pressure and due to the high temperatures, said cracks then leading to damage and ultimately to leaks in the tube.
Herein, tubes having a wall thickness of up to 5 mm to a maximum of 10 mm are referred to as thin-walled tubes. The VD TÜV (German Technical Control Association) material data sheet 533/2 does not provide for a subsequent heat treatment in TIG (tungsten inert gas) welded thin-walled tubes having a wall thickness of ≦10 mm. Hardness increases in welding could indeed be eliminated by a subsequent heat treatment at a tempering temperature; however, it has been found that this is not only expensive but may also lead to the formation of cracks and to a distortion of the heat-treated components. In large boilers, this method is not feasible.
Until now, another solution that has been considered is welding above the martensite start temperature. However, this temperature is very high and thus this suggestion has also been unsuitable in practical applications. This method results in a super-heated structure and is accompanied by a deterioration of the material properties.
Consequently, it is the object of the present invention to provide a welding method for welding tube wall registers that is also suitable for air-hardening steel alloys and, in particular, for materials T23 and T24 and, in particular, for thin-walled tubes.